The present invention relates to a porous metal-oxide thin film, and a method of forming the film on a glass substrate. The film-forming method belongs to the sol-gel process.
It is known that a porous metal-oxide thin film on a glass substrate is effective in reducing refractive index and thus in providing a low-reflectance glass plate. A porous metal-oxide thin film has a very large surface area, due to its porosity. Therefore, when it is used as a sub-layer of a multilayered film, the contact area between the sub-layer and another layer on the sub-layer becomes very large, and thus adhesion therebetween is much improved. With this, the multilayered film becomes much improved in abrasion resistance and durability.
There is known a method for preparing a porous glass such as Vycor (a trade name of Corning Co.) by phase separation. In this method, a sodium borosilicate glass is separated into an acid-soluble phase rich in Na.sub.2 O.B.sub.2 O.sub.3 and an acid-insoluble phase rich in SiO.sub.2, and then the acid-soluble phase is removed by an acid. With this, a porous silica glass is formed. However, when this method is used for forming a porous glass film, the film thickness tends to become too thick.
Furthermore, there is known a method for forming a porous metal-oxide film on a glass substrate, by etching. In this method, at first, a metal oxide film is formed on a glass substrate. Then, the surface of the metal oxide film is etched by hydrofluoric acid, fluorine nitrate or the like, to make the surface porous. However, hydrofluoric acid and fluorine nitrate are very hazardous against human body. Therefore, these compounds must be handled very cautiously. This lowers the production efficiency. Furthermore, the etching step increases the production cost.
There is known another method for forming a porous metal-oxide film on a glass substrate. In this method, at first, an organic polymer is added to a metal alkoxide solution (sol), to prepare a coating solution. Then, a glass substrate is coated with the coating solution. Then, the thus coated glass substrate is heated at a temperature not lower than the thermal decomposition temperature of the organic polymer, to remove the organic polymer and thus to make the metal-oxide film porous. However, during this heating, the metal oxide film may contract very much. This leads to the occurrence of cracks thereon. In order to prevent this, the amount of the organic polymer may be reduced. However, as the amount is reduced, micro-pores (micro-pits) making the film porous tend to disappear during the contraction (densification) of the film. With this, the surface of the metal oxide film may not become porous.
There is still another method for forming a porous metal-oxide film on a glass substrate, which is disclosed in U.S. Pat. No. 5,403,368 corresponding to Japanese Patent Kokai Hei 5-147976. This method belongs to the sol-gel process. In this method, at first, a coating solution is prepared by mixing at least two sols having different average molecular weights with a solvent. Then, a glass substrate is coated with the coating solution, so as to form thereon a sol film. Then, the thus coated glass substrate is heated, so as to transform the sol film to a porous gel film then to a porous metal-oxide film. However, according to this method, it is necessary to use certain special starting materials. Furthermore, from a standpoint of pot life of the sols, it may not necessarily easy to produce porous metal-oxide films having the same surface morphology.